1. Field of the Invention
The present invention generally relates to an aperture antenna for use in microwave band communication, radar, or the like and, more particularly, to the improvement of an offset antenna having a circular aperture.
2. Description of the Prior Art
A conventional offset antenna has such a structure as that disclosed in, e.g., Japanese Utility Model Unexamined Publication No. 27609/84. FIG. 1 is a side elevational view showing such conventional offset antenna, FIG. 2 is a front view thereof, and FIG. 3 is a partial perspective view. In FIGS. 1 to 3, a reflecting mirror 1 has a shape which is obtained by cutting with a plane P a paraboloid of revolution in which a focal point is set to F and a rotary axis is set to A--A, the plane P being inclined at an angle of .phi. to the rotary axis A--A. A primary radiator 2 consists of, e.g., a conical horn. The center of the phase of the radiation radio wave of the primary radiator 2 coincides with the focal point F of the reflecting mirror 1. A cylindrical side plate 3 is disposed around the reflecting mirror 1. A cut portion 4 is formed at the bottom of the side plate 3 as shown in FIG. 3. The hole 4 is covered by a box 5. An edge 8 is formed in the coupling portion of the side plate 3 and box 5. The primary radiator 2 is disposed in the box 5. The reflecting mirror 1 is attached to the aperture on one side of the side plate 3. The radio waves radiated from the primary radiator 2 are reflected by the reflecting mirror 1 and transmitted along a transmission path 7. The aperture of the side plate 3 through which the radio waves are emitted is covered by a radome 6 made of a dielectric material.
The conventional offset antenna is constituted in the manner explained above. When considering this antenna as a transmission antenna, the radio waves which are radiated from the primary radiator 2 are transmitted as shown by the transmission path 7. Namely, they are radiated as spherical waves whose centers are located at the center of the radiation radio wave phase of the primary radiator 2, i.e., at the focal point F. These radio waves are reflected by the reflecting mirror 1 and converted into plane waves, thereby forming a sharp beam in the forward direction of the antenna. When rain droplets or snow is deposited on the aperture of the primary radiator 2 the amplitude distribution and phase distribution of the radio waves radiated from the primary radiator 2 change, so that the inherent sharpness of the beam deteriorates and the radio waves are reflected in inappropriate directions. To avoid such problems, the radome 6 has a sealed structure together with the reflecting mirror 1, side plate 3 and box 5, thereby preventing rain or snow from entering the inside of the box and being deposited on the primary radiator 2. Further, the hole 4 of the side plate 3 is arranged so as not to block the radio waves propagating along the transmission path 7 from the primary radiator 2 to the forward direction of the antenna through the reflecting mirror 1. The radome 6 is also formed of a thin dielectric film which is thin enough in comparison with the wave length to minimize the reflection of the radio waves when they pass through the radome 6.
According to the offset antenna constituted in consideration of the foregoing points, deterioration in side lobe and reduction in gain due to blocking do not occur, unlike the parabolic antenna or Cassegrain antenna in which blocking is inherently experienced. Therefore, such an offset antenna has good characteristics and may be used for high density communications or satellite communications.
However, with respect to the hole 4 of the side plate 3 which is necessary in such conventional offset antennas, there are a few problems to be considered in terms of electrical characteristics and mechanical strength.
First, as for the electrical characteristics, when the hole 4 is considered from the viewpoint of geometrical optics, it is sufficient to form the hole with a shape corresponding to a intersecting line formed by cutting with the side plate 3 a circular cone which is generated by selecting the focal point F and the periphery of the reflecting mirror 1 as a vertex and a generating line, respectively. However, in practice, since the wavelength of radio waves is a few centimeters and the circular cone formed by the radio waves has a wave-like extension, the hole 4 must be larger than the intersecting line. In particular, the hole 4 needs to be formed with a sufficient size near the primary radiator 2. In practical use, as shown in FIG. 3, the hole 4 is generally formed so as to have substantially the same size as the box 5. Further, the hole 4 forms an edge 8 between the hole 4 and the side plate 3 and thus has this edge 8, as well as the outer periphery of the reflecting mirror 1, the periphery being an inherent edge of the antenna. The deterioration in side lobe characteristics increases due to edge diffraction or edge scattering. Therefore, it is necessary to attach a radio wave absorbing material or the like to the antenna.
Next, in regard to mechanical strength, the continuity of the side plate 3 as a cylindrical shell is interrupted by the hole 4. Further, at the portion where the side plate 3 and box 5 are coupled, the curvature of the shell and its direction suddenly change, so that an out-of-plane bending moment is generated in this portion. Therefore, countermeasures need to be taken, for example, to increase the thickness of the plate or to reinforce this portion. On the other hand, when the antenna is exposed to a strong wind such as are experienced in typhoons or the like, the box 5 disturbs the wind currents and increases the wind load. It is therefore necessary to construct a shape adapted to reduce the influence played by the wind on the basis of experiments using a wind tunnel or the like so as to reduce wind disturbance and to improve the strength of each section on the basis of the results of such experiments.
These are not the critical problems experienced with an offset antenna and the relevant countermeasures are taken to prevent deterioration in the inherent electrical and mechanical characteristics of the offset antenna. However, in order to improve those characteristics a radio wave absorbing material needs to be used, the plate thickness needs to be increased, and so forth, which means that the bulk of the antenna is increased. Further, with respect to the shape of the hole, the position at which the radio wave absorbing material is attached and the shape and selection of the box need to be determined by developing a procedure that utilizes significant experimental factors. This makes it difficult for a cheap offset antenna having good characteristics to be realized.
A second example of the conventional offset antenna is shown in, e.g., Japanese Patent Publication No. 31345/78. FIG. 4 is a side elevational view showing such an offset antenna and the front view and a partial perspective view thereof are as shown in FIGS. 2 and 3 (except that F.sub.2 is used in place of F shown in FIG. 2).
In these diagrams, the main reflecting mirror 1 has a shape which is obtained by cutting with a plane P a paraboloid of revolution in which the focal point is set to F.sub.2 and the rotary axis is set to A--A, the plane P being inclined at an angle of .phi. relative to the rotary axis A--A. A subreflecting mirror 9 has a shape constituting a part of an ellipsoid of revolution in which the conjugate focal points are set to F.sub.1 and F.sub.2 and the rotary axis is set to B--B. The phase center of the radiation radio wave from the primary radiator 2 coincides with F.sub.1 as one of the focal points of the sub-reflecting mirror 9. The primary radiator 2 consists of, e.g., a conical horn. The other compositional elements are similar to those employed in the offset antenna of the first conventional example which was described above with reference to FIGS. 1 to 3. When the offset antenna shown in FIG. 4 is used as a transmission antenna, the radio waves radiated from the primary radiator 2 are transmitted along the transmission path 7. Namely, the radio waves are radiated as spherical waves whose centers are located at the phase center of the radiation radio wave from the radiator 2, i.e., at the focal point F.sub.1. These radio waves are reflected by the sub-reflecting mirror 9 and transmitted via the focal point F.sub.2. Then, they are reflected by the main reflecting mirror 1 and converted into plane waves, thereby forming a sharp beam at a position in the formed direction of the antenna. Since the offset antenna in the second conventional example operates in a manner similar to the first conventional example, those with ordinary skill in the art will readily understand it without the need for further description.
There are thus problems to be considered in terms of the foregoing electrical characteristics and mechanical strength, similar to the case of the first conventional example.